Process for manufacture of coated superconductive ribbons



June 27, H. COHEN v FOR MANU ACTURE OF COATED PROCESS S 'RCONDUCTIVE RIBBONS led Feb. 15, 1963 CLAD TIN I2 l4 K I/ 1 BORON NITRIDE SECTHDN f; a

INVENTOR.

HARVEY M. COHEN BY United States Patent 3,327,370 PROCESS FOR MANUFACTURE OF COATED SUPERCONDUCTIVE RIBBONS Harvey M. Cohen, Waltham, Mass, assignor, by mesne assignments, to National Research Corporation, a corporation of Massachusetts Filed Feb. 15, 1963, Ser. No. 258,758 6 Claims. (Cl. 29155.5)

The present invention relates to improvements in heat treating techniques for producing hard superconductors by diffusion. The copending application of Allen and Stauifer Ser. No. 225,784, filed Sept. 24, 1962 and now Patent No. 3,296,684, discloses the technique of coating niobium ribbon with tin and winding the coated ribbon into a tight spool for placement in a heat treating furnace to produce Nb Sn at the niobium-tin interface.

It is an object of the present invention to provide an improvement in the techniques of Allen and Stauffer which will expedite the heat treatments.

It is a further object of the invention to provide a method of treating the coated ribbon to enhance unwinding of the spool after heat treatment.

It is a further object that the improved treatment will not cause any loss of superconductivity in the finished product.

These and other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention comprises the process of applying to the tin-coated ribbon a coating of boron nitride. It has been found that this material effectively protects the tin-coated niobium during heat treatment. It has also been found that the superconductivity characteristics of the resultant product is high. The invention also comprises the suspension of boron nitride in a volatile carrier to facilitate its application, as by spraying. The carrier is acetone, which, unlike other organic materials, readily provides suspensions of boron nitride.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is a diagrammatic, sectional view of a coated niobium ribbon;

FIG. 2 is a diagrammatic, sectional view of the same ribbon at a later stage of treatment; and

FIG. 3 is a diagrammatic, isometric view showing the manner in which ribbon is wound for heat treatment.

FIG. 3A is a cross-section taken in the direction AA, as indicated in FIG. 3.

Referring now to FIG. 1, there is shown a cross-section of coated niobium ribbon 10. The tin coating 12 is applied to the base 14 by cladding and prolonged rolling as taught in the copending application of Allen, Das and Stauifer, Ser. No. 207,320, filed July 3, 1962 and now Patent No. 3,218,693. In a preferred embodiment of the invention, the ribbon is folded in the following manner so that the niobium sandwiches the tin. Two sides of the ribbon are folder upwardly to form the legs of a U shape. These legs are then col-lapsed on the base to produce the resultant ribbon shown at 10 in FIG. 2. Such a ribbon can be wound tightly onto a spool and heat treated to produce Nb Sn in accord with the above said application, Ser. No. 225,784, of Allen and Stauifer. However, difficulty is sometimes encountered in unwinding the spool due to sticking of adjacent turns. Where such difficulty is encountered, the critical current carrying capacity of the finished product will be low.

In accordance with the present invention, a coating 16 of boron nitride is applied to the ribbon 10 of FIG. 2. The ribbon is then wound into a spiral as shown in FIG.

3,327,370 Patented June 27, 1967 ice 3 and heat treated. Niobium spool 20 supports the spiral in the furnace.

The boron nitride may be applied by painting it on from a water slurry, using well known dispersion agents. However, it is preferred to apply a suspension of boron nitride in a volatile, organic carrier by a spray or a slurry. In accordance with the present invention, this suspension is made possible by the use of acetone as a carrier. Suspensions of boron nitride in other carriers are maintained only by continued violent agitation. Acetone suspends the boron nitride for long periods of time with minimal agitation at infrequent intervals. The suspension may be sprayed through conventional paint spray guns. The boron nitride to be used in suspension should be 325 mesh. It is preferred to provide a stirrer to the suspension. However, agitation may be applied by shaking the suspension.

It is shown by the following non-limiting examples that boron nitride does not have any adverse effects on the superconductivity of the finished product. On the other hand, superconductivity is limited when boron nitride is replaced by graphite, which has a similar structure.

Example 1 Several ribbons, similar to that shown in FIG. 1 were prepared in lengths of from 20 to 50 feet. They were all folded as shown in FIG. 2 to produce cross-section dimensions of 3 mils by 65 mils inch). The ribbons were coated with boron nitride by making a slurry of 325 mesh particles in acetone and passing the ribbons through the slurry. The coated ribbons were wound into tight spirals and then heated at 110 C. overnight to volatilize the acetone and then heated in argon furnaces for one hour at temperatures from 967 C. to 975 C. After heat treatment, the ribbons unwound spontaneously with no evidence of sticking. Tests for critical current at liquid helium temperatures and under a magnetic field of 20 kilogauss, applied transversely normal to the plane of the ribbon, gave results from to 108 amperes.

Example 2 A ribbon similar to that shown in FIG. 1 was prepared in a length of several feet and /s" wide. The ribbon was then coated with boron nitride without folding. The coating was accomplished as in Example 1. After removing the acetone carrier, as in Example 1, the ribbon was heated in an argon furnace for 2 hours at 955 C. After heat treatment, the ribbon was unwound easily and tested for critical current as in Example 1. It had a critical current in excess of 112 amperes under an external field of 20 kilogauss.

Certain changes may be made in the above process. For instance, the tin coating may be done by vapor deposition or electroplating. Niobium ribbon may be folded around tin ribbon and coated with boron nitride to produce a ribbon having a cross-section similar to that shown in FIG. 2. Tin coated wires may be protected by boron nitride in the same manner as ribbons. The invention is also applicable to plate and rods as well as foil ribbons and wires.

Where the tin coated material is heat treated without winding upon itself, the boron nitride coating serves to protect the tin from contamination at the elevated heat treatment temperatures.

Since certain changes can be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In the method of making superconductive ribbons comprising the steps of coating a niobium ribbon with 3 tin and diffusing the tin into the niobium by heat treatment at temperatures of about 800-1100 C. to form an Nb Sn layer, the improvement comprising the steps of coating the ribbon with a layer of boron nitride and winding the ribbon into a tight spool with adjacent turns bearing against each other.

2. The method of claim 1 wherein the tin is coated on one side of the ribbon only and the ribbon is folded, so that the niobium sandwiches the tin, prior to coating with boron nitride and winding.

3. The method of claim 2 wherein the folding is accomplished by first turning two sides up to form the legs of a U shape in cross-section, with the tin-coated side being inside the U shape, and then collapsing the legs upon the base to form a resultant ribbon with outer niobium surfaces and tin trapped inside.

4. The method of claim 1 wherein the tin is coated on the niobium by cladding.

5. The method of claim 1 wherein the boron nitride is applied by spraying it in suspension with a volatile carrier and removing the carrier from the ribbon by heating.

6. The method of claim 5 wherein the volatile carrier is acetone.

References Cited UNITED STATES PATENTS 1,063,483 6/1913 Weintraub 1175.1 2,201,049 5/1940 Moore 1175.1 2,726,160 12/1955 Ueltz 10638.22 2,731,360 1/1956 Love 117-49 ALFRED L. LEAVITT, Primary Examiner. WILLIAM L. JARVIS, Examiner. 

1. IN THE METHOD OF MAKING SUPERCONDUCTIVE RIBBONS COMPRISING THE STEPS OF COATING A NIOBIUM RIBBON WITH TIN AND DIFFUSING THE TIN INTO THE NIOBIUM BY HEAT TREATMENT AT TEMPERATURES OF ABOUT 800-1100*C. TO FORM AN NB3SN LAYER, THE IMPROVEMENT COMPRISING THE STEPS OF COATING THE RIBBON WITH A LAYER OF BORON NITRIDE AND WINDING THE RIBBON INTO A TIGHT SPOON WITH ADJACENT TURNS BEARING AGAINST EACH OTHER. 